1. Field of the Invention
The present invention relates to a timing control valve for use in the automatic transmission control apparatus for vehicles, which is provided in the hydraulic control circuit for controlling the operation timing of brakes, clutches, etc. in the transmission unit so that they can be operated at appropriate timings at each shifting.
2. Description of the Prior Art
When a shifting from a higher speed gear ratio to a lower speed gear ratio (downshifting) takes place, brakes, clutches or one-way clutch on the one hand, which are being engaged in a higher speed gear ratio ("shift range" or "speed" hereinafter) and are to be disengaged or released in a lower speed shift range, and brakes, clutches or one-way clutch on the other hand, which are being released in the higher speed shift range and are to be engaged in the lower speed shift range are engaged and or released at appropriate timings. However, in order to permit a smooth downshifting, intervals or timing lags between those timings should desirably tend properly to increase with an increasing engine speed or with increase in throttle opening degree which is in correlation with the engine speed. It has been known that to properly increase those timing lags is particularly important in clutch-to-clutch downshifting without the aid of an intermediate one-way clutch.
A conventional timing valve as shown in FIG. 1 which provides timing lag control functions as described above. The timing valve shown in FIG. 1 is actuated by a spring 52 backing one side of the spool 51, and by a thereagainst counteracting operating oil pressure PC.sub.2 from a hydraulic servo unit applied on a piston portion 53 of the spool 51. The pressure PC.sub.2 causes the clutch C.sub.2 to be engaged in a higher speed shift range and to be disengaged in a lower speed shift range, so that the timing valve controls oil pressure supply to the brake B.sub.1 permitting the brake B.sub.1 to be engaged after a certain timing lag or delay when a downshifting to the lower speed shift range takes place. As the operating hydraulic (oil) pressure PC.sub.2, which is proportional to a line pressure PL introduced into a hydraulic circuit through a pressure regulating valve for regulating an output pressure of an oil pump driven by the engine, rises in accordance with increase in the engine speed or throttle opening degree, the operating oil pressure PC.sub.2 which lies at an elevated state will take a longer time before being wholly exhausted, thus allowing an increased timing lag between releasing step of the clutch C.sub.2 and subsequent engaging step of the brake B.sub.1. However, according to the conventional timing valve, it is impossible to provide a sufficient timing lag change to meet the required timing lag corresponding to the increase in the throttle opening degree.